This invention relates to a horizon sensor for indicating orientation by sensing a line of discontinuity in optical radiation primarily in the infrared range between a reference planet and outer space utilizing a single set of optics and one scanning element to provide two axis information.
Horizon sensors are devices which are mounted in satellites, missiles, space probes, extremely high flying aircraft and the like which sense the horizon of a reference planet and produce a signal which indicates orientation with respect thereto. Almost all unmanned satellites employ horizon sensors for attitude control and guidance of the satellite. The attitude of the satellite is determined by its position with respect to three axes at right angles to each other. Two of these axes are in a plane at right angles to a projected radius of the earth passing through the satellite and the third coincides with such radius. The plane in which the first two axes lie is parallel to the earth's horizon and it is with information on the attitude with respect to these two axes that the present invention deals. Once the orientation of the satellite with respect to these two axes is determined, orientation around the other axis can be determined by other means utilizing gyroscopes, or observation of heavenly bodies.
One very successful type of horizon sensor is shown and described in U.S. Pat. No. 3,020,406, which is assigned to the assignee of the present invention, which utilizes the horizon representing a line of discontinuity between the earth's atmosphere and outer space. This line of discontinuity is characterized by a large difference in infrared radiation between outer space which is cold providing very little infrared radiation whereas the earth is considerably warmer and provides a large amount of infrared radiation as compared with space. Accordingly, the horizon represents a sharp line of discontinuity which provides an abrupt and sharp change in infrared radiation on either side of it. This characteristic is utilized by scanning an infrared detector in a conical scan pattern across the horizon and deriving an electrical output signal marking the line of discontinuity. Reference pulses are also generated as the scan passes predetermined points in the vehicle. The intervals between the horizon crossings and the reference pulses are compared to produce an output signal which provides information with respect to the attitude of the vehicle with respect to the horizon. The type of conical scan sensors as set forth in the aforesaid patent have proven extremely successful in space flight and are used on many manned and unmanned satellites.
The disadvantage of the aforesaid sensor resides in the fact that two sensor heads spaced 90.degree. apart on the satellite or space vehicle are required to provide two axis information thus one head is required to provide information for each axis. This increases the weight, power and cost of the system as well as providing more moving parts thereby increasing the chance of a malfunction based on a greater number of moving parts.